General Science

PHYSICS: HEAT

Overview

• Heat is the process of energy transfer from one system to another
• Units of heat: Joules (J), Calories, British Thermal Unit (BTU)
• Temperature is a measure of internal energy (enthalpy)
• Heat transfer can happen spontaneously only from a warmer to a colder body. Reverse heat transfer can only happen with the aid of an external source such as a heat pump.

Mechanisms of heat transfer

• Conduction is the most significant heat transfer mechanism in solids. It occurs as hot high energy molecules interact with neighbouring and transfer heat to them. Eg: heat transfer from one end of a metal rod to another
  
• Convection is most significant in liquids and gases. This happens when hot molecules move and transfer energy to other molecules. Eg: boiling of water. When water is heated on a stove, hot water from the bottom rises and displaces colder liquid which falls. 
  
• Radiation is the only form of heat transfer possible in the absence of a medium. Heat is transferred in the form of electromagnetic radiation. Eg: heat from the sun reaching the earth.
  

Heat transfer in everyday life

• Copper is used in construction of boilers and cooking utensils because it is a good conductor of heat
  
• Air is a poor conductor
  
• Wool and cotton are good insulators i.e. poor conductors. Their insulation arises mainly due to air spaces between molecules
  
• Double-walled glass doors with air between them are better insulators than windows with a single thick glass layers
  
• Eskimos live in snow huts because snow is a poor conductor of heat, and hence protects them from the extreme cold outside.
  
• Land and sea breeze, ocean currents are arise due to convection
  
• The boiling point of water at sea level and atmospheric pressure is 100C. When extra heat is added, it changes the phase of water from liquid to gas (water vapour). 
  

Thermometers

• Thermometers can be divided into two groups: 
  
  • Primary thermometers: measure temperature directly based on the property of matter. They are relatively complex and not used commonly. Eg: thermometers based on velocity of sound in gas, thermal noise of an electrical resistor etc.
    
  • Secondary thermometers: measure temperature relative to a pre-calibrated quantity. They are easy to use and used commonly.Alcohol thermometer, mercury thermometer, medical thermometer are all secondary thermometers
• In cold winter places, alcohol thermometers are used instead of mercury thermometers because the freezing point of alcohol is lower
• For extra-low temperature measurements (-200 C), Pentane is used
  
• Water is not suitable for use in thermometers because it freezes at 0 C and has irregular expansion
  
• Mercury is used for common medical thermometers because
  • It does not cling to glass and hence reading is easy
    
  • It is opaque and easily seen
    
  • Its expansion is uniform and hence calibration is easier
    
  • It is a better conductor of heat than alcohol and hence responds more rapidly to changes of temperature
    
  • It has low specific heat capacity and hence is more sensitive
    

Common appliances based on heat

• Solar cooker: is a box made of insulating material such as wood, cardboard etc. The box has a glass cover to retain heat inside by greenhouse effect. The inside of the box is painted black to increase heat absorption.
  
• Pressure cooker: Pressure cooker increases the boiling point of water by increasing pressure. When the boiling point of water increases, food cooks faster. Pressure cookers are especially essential in hill stations because at higher altitudes the boiling point of water decreases due to lower atmospheric pressure
  
• Refrigerator and Air-conditioner: are heat pumps that transfer heat from inside to the external environment. They use a refrigerantwhich is a compound that undergoes reversible phase change from gas to liquid. Common refrigerants include ammonia, sulphur dioxide, carbon dioxide and methane. The use of chlorofluorocarbons has been phased out due to concerns regarding depletion of the ozone layer.
  

CHEMISTRY: ELECTROLYTES

Electrolytes in the human body

• Electrolytes are required in the body to maintain balance between intracellular and extracellular liquids. In particular, it is important to maintain the osmotic gradient between inside and outside.
  
• Electrolyte balance is maintained by oral and intravenous intake
  
• Kidneys flush out excess electrolytes
  
• Dehydration and overhydration are caused by electrolyte imbalance
  
• Hormones that maintain electrolyte balance are antidiuretic hormone, aldosterone and parathyroid hormone
  
• The most common electrolyte in the body is salt (sodium chloride)

Functions of electrolytes in the body

• Maintain blood pH
  
• Muscle and neuron activation
  
• Hydration of the body
  

Other common applications of electrolytes

• Sports drinks
  
• Batteries
  
• Fuel cells
  
• Electroplating
  
• Capacitors
  

Sports Drinks

• Sports drinks replenish the body’s water and electrolyte levels after dehydration caused by exercise, vomiting, diarrhea etc.
  
• They are made of electrolytes containing sodium and potassium salts
  
• Examples of sports drinks: Glucon-D, Gatorade etc
  
• Simplest electrolyte drink that can be made at home is water + sugar + salt
  

Batteries

Battery	Electrode	Electrolyte	Other notes
Alkaline	Zinc, Manganese oxide	Potassium Hydroxide
Daniell cell	Copper, Zinc	Copper sulphate, zinc sulphate
Leclanche cell	Zinc, carbon	Ammonium chloride	Precursor of modern dry cell
Voltaic pile	Copper, zinc	Brine	First electric battery, invented in 1880
Zinc carbon	Zinc, carbon, manganese dioxide	Zinc chloride, ammonium chloride	Most common battery
Zinc chloride	Same as above	Zinc chloride	Improvement on zinc carbon battery
Lead-acid	Lead, lead dioxide	Sulphuric acid	Oldest rechargeable battery Used in vehicles as they provide high surge currents
Lithium-ion	Graphite, Lithium Cobalt oxide	Non-aqueous lithium salts	Rechargable Slow self-discharge, high energy to weight ratio
Nickel Cadmium	Nickel oxide hydroxide, cadmium		Rechargable Last longer, more stable than lithium ion
Fuel cell	Hydrogen (fuel), oxygen (oxidant)	Polymer membrane Aqueous alkaline solution	Consumes reactant from an external source High energy efficiency and high reliability No moving parts Used in space shuttles, submarines

Common electrolytes and their uses

Electrolyte	Uses	Other notes
Sodium chloride	Primary component of extracellular fluid Food preservative
Sodium hydroxide (caustic soda)	Manufacture of paper, soaps, detergents, drain cleaners Purification of drinking water
Silver nitrate	Photographic films Water disinfection (esp. on space shuttles)
Hydrochloric acid	Manufacture of PVC, household cleaners Food additives (like gelatin) Leather processing	Found naturally in gastric acid
Sulphuric acid	Lead-acid batteries Ore processing Fertilizer manufacture	Soluble in water at all concentrations One of the largest products of chemical industry
Nitric acid	Determining metal traces in solutions Wood finishing	Colourless when pure, yellows with age Highly corrosive
Acetic acid	Manufacture of soft drink bottles Photographic films Synthetic fibres and fabrics	Dilute acetic acid is called vinegar
Ammonium hydroxide (aqueous ammonia)	Cleaning agent
Calcium hydroxide (slaked lime or pickling lime)	Sewage treatment Whitewash, plaster, mortar Hair relaxers	Natural mineral form is called portlandite (rare mineral occurring in volcanic rocks)

BIOLOGY: BLOOD

Overview

• Blood is a specialized body fluid that delivers necessary substances to various cells (like nutrients and oxygen) and transports waste products away from those cells
• Blood accounts for 7% of human body weight
  
• The average human adult has a blood volume of approx. 5 litres
  
• Arteries carry inhaled oxygen-rich blood from the heart to the tissues, while veins carry carbon dioxide rich blood (de-oxygenated) from the tissues to the lungs to be exhaled
  

SEM image of a RBC, a platelet and a WBC (L to R)

Composition of blood

• Blood is made of plasma, Red Blood Cells, White Blood Cells (including leukocytes and platelets)
  
• Plasma constitutes about 54.3% of blood, RBCs 45% and WBCs about 7%
  
• RBCs contain hemoglobin and distribute oxygen to tissues
• Leukocytes attack and remove pathogens and provide immunity
  
• Platelets are responsible for clotting of blood
  
• Plasma is the blood’s liquid medium. It circulates dissolved nutrients and removes waste products. By itself, it is yellow in colour
  

Functions of blood

• Supply oxygen to tissues
  
• Supply nutrients such as glucose, amino acids and fatty acids
  
• Remove waste such as carbon dioxide, urea and lactic acid
  
• Provide immunity against pathogens
  
• Coagulation
  
• Transport hormones
  
• Regulate pH
  
• Regulate core body temperature
  

Colour of blood

• Colour is primarily determined by hemoglobin 
  
• Arterial blood is bright red, due to the presence of oxygen
  
• Venous blood is dark red, due to deoxygenation
  
• Blood in carbon monoxide and cyanide poisoning is bright red
• Blood of most molluscs (marine animals like squids, oysters, snails, octopuses etc) is blue due to the presence of copper containing protein hemocyanin
  

Blood Groups

Blood Group	Can donate to	Can receive from
A	A and AB	A and O
B	B and AB	B and O
AB	AB only	All groups
O	All groups	O only

Medical disorders related to blood

Disorder	Cause	Other notes
Bleeding		An adult can lose 20% of blood volume before the first symptom (restlessness) sets in
Dehydration	Loss of volume due to loss of water
Atherosclerosis	Reduced blood flow through arteries
Thrombosis	Coagulation of blood vessels
Hypoxia (lack of oxygen)	Narrowing of blood vessels Problem with pumping action of heart	Can lead to ischemia (tissue with insufficient blood) or to infarction i.e. necrosis (tissue death)
Anemia (insufficient RBC)	Bleeding, nutritional deficiencies
Sickle-cell disease	Mutation of hemoglobin leading to abnormal sickle shape of RBC	Sickle shaped RBCs do not have the flexibility to travel through many blood vessels Extremely painful disease with no known cure Found commonly in malaria-infested areas because sickle cells offer resistance to malaria
Leukemia	Abnormal proliferation of WBCs in the bone marrow
Hemophilia	Dysfunction of clotting mechanism	Lack of coagulation means simple wounds become life-threatening Causes hemarthosis (bleeding into joints), which is painful and crippling Linked to X chromosome Occurs usually in males only
Thrombophilia	Abnormal propensity to coagulate
Blood-borne infections	Infection by a disease-carrying vector	Examples: HIV, Hepatitis, Malaria
Carbon monoxide poisoning	Carbon monoxide binds to hemoglobin preventing oxygen transport	Body tissues die due to lack of oxygen